Add entry in release notes

Author: mjklemm

flang/docs/ReleaseNotes.md

@@ -25,6 +25,7 @@ page](https://llvm.org/releases/).
 ## Major New Features
 
 * Initial support for VOLATILE variables and procedure interface arguments has been added.
+* Support for LOWER= argument for C_F_POINTER.
 * OpenMP support is stable and no longer considered experimental. All of OpenMP 3.1 is
   supported, along with much of OpenMP 4.0 and some parts of later standards.